Assuming good faith is a fundamental principle on Wikipedia. It is the assumption that editors' edits and comments are made in good faith. Most people try to help the project, not hurt it. If this were false, a project like Wikipedia would be doomed from the beginning.
When disagreement occurs, try to the best of your ability to explain and resolve the problem, not cause more conflict; and give others the opportunity to do the same. Consider whether a dispute stems from different perspectives, and look for ways to reach consensus.
When doubt is cast on good faith, continue to assume good faith yourself where you can. Be civil and follow dispute resolution processes, rather than attacking editors or edit warring with them. If you wish to express doubts about the conduct of fellow Wikipedians, please substantiate those doubts with specific diffs and other relevant evidence, so that people can understand the basis for your concerns. Although bad conduct may seem to be due to bad faith, it is usually best to address the conduct without mentioning motives (which mention would tend to exacerbate resentments all around).
Be careful about citing this principle too aggressively. Just as one can incorrectly judge that another is acting in bad faith, so too can one mistakenly conclude that bad faith is being assumed, and exhortations to "Assume Good Faith" can themselves reflect negative assumptions about others if a perceived assumption of bad faith was not clear-cut.